Laser initiated non-linear fuel droplet ignition

ABSTRACT

Method and apparatus for igniting an air/fuel spray (26) comprised of fuel droplets. The apparatus includes a coherent optical source (12) for introducing at least one pulse of coherent radiation into the air/fuel spray. The pulse generates free electrons and initiates a development of a plasma within the air/fuel spray. The coherent source maintains the pulse of coherent radiation and pumps the developing plasma to higher energies. The pulse is terminated at a time after the plasma has reached a predetermined energy and before ignition of the air/fuel spray. The non-linear ignition system of the invention employs a gas/vapor interface region at a fuel droplet surface and an electric field that extends from and exists outside of a fuel droplet. Free electrons are accelerated to higher energies by the electric field surrounding the fuel droplet. The accelerated electrons initiate a breakdown near adjacent fuel droplets and the liberation of further free electrons. In a short period of time an avalanche process occurs that creates a high density of free electrons and ions which results in the formation of a plasma.

FIELD OF THE INVENTION

This is a continuation Ser. No. 07/957,613 filed on Oct. 6, 1992 nowU.S. Pat. No. 5,404,712.

This invention relates generally to ignition systems and, in particular,to the ignition of fuel droplets by a laser initiated process.

BACKGROUND OF THE INVENTION

The laser ignition of fuel droplets, such as those found in thecombustion chamber of a gas turbine engine, provides several significantadvantages over conventional spark plug type ignitors. For example, witha laser ignitor the combustion process can be started in a more optimumchamber position. Spark ignitors are typically positioned at aperipheral, non-optimal position of a combustion chamber, while the fuelspray to be ignited is located in a central portion adjacent to a fuelinjector. Also, a laser ignition system that operates outside thecombustion chamber, with an appropriate optical coupling into the fuelspray, is not subject to the type of degradation experienced byinternally mounted spark ignitors. This degradation of spark ignitors isknown to cause eventual failure after prolonged use.

In U.S. Pat. No. 4,947,640, issued Aug. 14, 1990, entitled "Gas TurbineEngine Photon Ignition System", two of the present inventors describemethod and apparatus for igniting a hydrocarbon fuel that is comprisedof droplets of hydrocarbon fuel. The hydrocarbon fuel is provided as anair/fuel spray. Electromagnetic radiation having wavelengths primarilywithin a range of approximately 185 nm to approximately 400 nm (UV) isgenerated and directed into the air/fuel spray. The droplets absorb theenergy, are heated, fragmented and ignited. The use of electromagneticradiation within the ultraviolet region is shown to be beneficialbecause of a high absorption of radiation within this wavelength rangeby hydrocarbon fuels such as JP-4 and JP-5.

The following U.S. and foreign patents are cited as relating to theignition of and/or the preconditioning of fuels with electromagneticenergy:

U.S. Pat. No. 4,035,131, issued Jul. 12, 1977, entitled "Control of theInitiation of Combustion and Control of Combustion" by A. E.Cerkanowicz; U.S. Pat. No. 4,726,336, issued Feb. 23, 1988, entitled "UVIrradiation Apparatus and Method for Fuel Pretreatment EnablingHypergolic Combustion" by L. O. Hoppie et al.; U.S. Pat. No. 3,258,910,issued Jun. 8, 1962, entitled "Fiber Optic Ignition" by R. J. Seymour;U.S. Pat. No. 3,473,879, issued Oct. 21, 1969, entitled "Shock WaveBurner" by B. Berberich; U.S. Pat. No. 3,861,371, issued Jan. 21, 1975entitled "Ignition System for Engine" to J. Gamell; U.S. Pat. No.4,416,226, issued Nov. 22, 1983, entitled "Laser Ignition Apparatus foran Internal Combustion Engine" by M. Nishida et al.; U.S. Pat. No.4,434,753, issued Mar. 6, 1984, entitled "Ignition Apparatus forInternal Combustion Engine" to Mukainakano et al.; and two U.K. Patentsto D. Brown, both entitled "Ignition Systems" specifically: 1,236,561,published Jun. 23, 1971, and 1,360,196 published, Jul. 17, 1974.

The following two technical reports are cited for teaching the ignitionof premixed flowing gases with electromagnetic energy: Brad E. Forch etal., Technical Report BRL-TR-27409, U.S. Army Ballistic ResearchLaboratory, entitled "Photochemical Ignition Studies. II Oxygen-AtomTwo-Photon Resonance Effects", June, 1986; and Andrezik W. Miziolek etal. Technical Report BRL-TR-2644, U.S. Army Ballistic ResearchLaboratory, "Photochemical Ignition Studies. I. Laser Ignition ofFlowing Premixed Gases", February 1985.

Finally, in U.S. Pat. No. 4,302,933, issued Dec. 1, 1981, entitled "JetEngine Augmentor Operation at High Altitudes" by Marvin M. Smith thereis disclosed a pulsed CO₂ TEA laser that is employed to generatelaser-supported absorption (LSA) waves within a jet engine augmentor.The LSA waves are initiated by a laser beam reflecting off of targets,such as fuel droplets, in the eye of a cyclonic air action in arecirculation zone of the augmentor. This is said to cause the emissionof electrons which serve as priming electrons to break down air into aplasma of high temperature (10,000° to 20,000° K.). The LSA waves aresaid to be initiated by directing a 10.6 micron wavelength convergingbeam having a minimum intensity of 6×10⁸ W/cm². The air breakdown issaid to proceed via inverse bremsstrahlung heating. It is stated thata/hot air plasma of 1-2 ev is formed which propagates back up the laserbeam away from the formed plasma where most of the laser beam energy isabsorbed. The system of Smith is said to measure the ambient airpressure and to vary the pulse repetition rate of the laser such thatthe fuel-air ratio in the wave combustion is continuously maintained ata high thermal efficiency. Smith also makes reference to the use of"additives" that may be used in conjunction with the fuel.

The teaching of Smith does not address the problem of low powered andlight-weight laser ignition systems for use within a gas turbine enginecombustor. As is evident from his disclosure, Smith envisions a highpowered laser source to generate LSA waves within the engine augmentor.

However, one important criteria for an aircraft laser ignition system isthat the system be a low powered system having a small physical size andweight. It is thus one object of this invention to provide for a laserinitiated ignition of a fuel spray within a combustion chamber with alow powered laser source.

It is a further object of this invention to provide for a laserinitiated non-linear ignition process of a fuel spray within acombustion chamber of a gas turbine engine.

SUMMARY OF THE INVENTION

The foregoing and other problems are overcome and the objects of theinvention are realized by method and apparatus for igniting an air/fuelspray comprised of fuel droplets. The apparatus includes a laser opticalsource for introducing at least one pulse of coherent optical radiationinto the air/fuel spray. The radiation source provides a light pulsethat generates free electrons and initiates a development of a plasmawithin the air/fuel spray. The source maintains the pulse of laserradiation and pumps the developing plasma to higher energies. The pulseis terminated at a time after the plasma has reached a predeterminedenergy and before ignition of the air/fuel spray.

In accordance with the invention, the pulse width of the incident laserbeam may be made relatively short as compared to the amount of time toachieve ignition, thereby achieving low powered operation. The processof plasma generation occurs within several tens of nanoseconds after thelaser pulse is introduced into a combustion chamber, such as acombustion chamber associated with a gas turbine engine. The actual,global ignition of the air/fuel spray may not occur for up to somenumber of microseconds after the pulse is first applied. However, thepulse width of the laser source can be made significantly shorter thanthe amount of time that elapses between plasma development and theignition of the air/fuel spray. That is, the pulse of coherent, laserradiation is introduced and removed within a period of time that is lessthan a time required to achieve ignition. In accordance with theteaching of the invention, the pulse of coherent radiation initiates thedevelopment of the plasma, and then pumps the plasma to an energy atwhich the plasma becomes self-sustaining. This advantageously providesfor a relatively low powered operation as compared to the prior art.

The non-linear ignition system of the invention employs a gas/vaporinterface region at the fuel droplet surface and an electric field thatextends from and exists outside of a fuel droplet. The shape and extentof the electric field is a function of the index of refraction of thedroplet at the laser wavelength, the droplet size, the dropletcomposition, and other factors. Free electrons are accelerated to higherenergies by the electric field surrounding the fuel droplet. Theseaccelerated electrons initiate a breakdown near adjacent droplets andthe liberation of additional free electrons. In a short period of timean avalanche process occurs that creates a high density of freeelectrons and ions which form a plasma.

In accordance with a method of igniting an air/fuel spray comprised offuel droplets there are disclosed the steps of (a) providing theair/fuel spray within a combustion chamber; (b) introducing at least onepulse of coherent radiation into the air/fuel spray, the at least onepulse interacting with free electrons and initiating a development of aplasma within the air/fuel spray; (c) maintaining the at least one pulseof coherent radiation for pumping the developing plasma; and (d)terminating the at least one pulse of coherent radiation at a time afterthe plasma has reached a predetermined energy and before ignition of theair/fuel spray.

BRIEF DESCRIPTION OF THE DRAWING

The above set forth and other features of the invention are made moreapparent in the ensuing Detailed Description of the Invention when readin conjunction with the attached Drawing, wherein:

FIG. 1 is a block diagram showing the laser ignition system of theinvention coupled to a combustion chamber of a gas turbine engine forigniting a fuel spray therein;

FIG. 2 is a graph showing, as a function of time, the energy of freeelectrons of a plasma that is laser-initiated within a combustionchamber;

FIG. 3 is an enlarged view, not to scale, of a fuel droplet and theenhancement region surrounding the droplet; and

FIG. 4 is an illustration of a focal region of a laser beam.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 there is shown a laser ignition system constructedand operated in accordance with the invention. The system includes acoherent source or laser 12 having a pulse controller (PC) 14 coupledthereto. It should be understood that the coherent source 12 mayinclude, if desired, frequency doubling or tripling apparatus,Q-switching apparatus, and/or other optical devices. An output of thelaser 12 is optically coupled to a radiation delivery means, such as,for example, an optical fiber 16 which delivers the laser radiation to afocussing element 18. The focussing element 18 provides a focussed beam20 of laser radiation. The focussed beam 20 is directed to within acombustion chamber 22. The combustion chamber 22 is preferably thecombustion chamber of a gas turbine engine, although other combustionchamber embodiments also benefit from the teaching of the invention. Afuel injector 24 is provided for introducing an air/fuel spray 26 intothe combustion chamber 22. The air/fuel spray 26 includes fuel dropletshaving a diameter within a range, typically, of 20 micrometers to 1000micrometers or greater. The mean diameter of fuel droplets within theair/fuel spray 26 is a function of several parameters, including fueltemperature and the design of the injector 24. The focussed beam 20 ispositioned such that the focal point 20a is located within the fuelspray 26. Positioning the focal point 20a within the fuel spray 26increases the probability that one or more fuel droplets will be at thefocal point 20a.

In FIG. 1 the focussing element 18 may be a lens, or may be embodied ina self-focussing type of optical fiber. Also, it is within the scope ofthe invention to provide a plurality of fiber optic conductors 18 forproviding a plurality of focussed beams 20 within the fuel spray 26,thereby increasing the probability that the nonlinear ignition processwill be initiated.

Also, it is within the scope of the invention to provide a feedbackmechanism for varying a pulse rate and/or a pulse width of the laser 12.In FIG. 1 this feedback mechanism includes a silicon photodiode 28 thatis responsive to wavelengths associated with the flame within thecombustion chamber 22. The silicon photodiode 22 generates a signalindicating that ignition has been achieved, the signal being fed back tothe pulse controller 14 for disabling the pulsing of the laser 12. Thisfeedback mechanism may also be employed to advantage if the flame withinthe combustion chamber 22 is lost, so as to reinitiate the ignitionprocess by enabling the laser 12 to generate pulses. Manual control ofthe laser 12 operation is generally always provided.

In accordance with the invention the ignition of the air/fuel spray 26proceeds by a non-linear process, as opposed to a linear process. In alinear process, the fuel droplets absorb the electromagnetic radiationand are heated thereby to an ignition temperature. Although providingignition, the linear system typically requires significantly more inputpower than the non-linear process described herein.

In contradistinction to the linear system, the non-linear ignitionsystem of the invention relies upon a gas/vapor interface region at thedroplet surface and upon an electric field that extends from and existsoutside of a fuel droplet. This electric field is generated by theinteraction of the droplet and the laser energy contained within thepulsed beam 20. The shape and extent of this electric field is afunction of the index of refraction of the droplet at the laserwavelength, the droplet size, the droplet composition, and otherfactors.

Referring to FIG. 2 it can be seen that a pulse of coherent radiationhaving a predetermined pulse width and power density is introduced intothe combustion chamber 22 at time (t_(o)). The focussed pulse initiatesa breakdown process, including the formation of a shock wave. Initially,typically but a few free electrons are generated from the gas/vaporsurrounding a fuel droplet. These free electrons are then accelerated tohigher energies by the aforementioned electric field surrounding thefuel droplet. These accelerated electrons initiate a breakdown in thegas/vapor surrounding adjacent droplets and the liberation of furtherfree electrons. Initially, free electron movement is approximatelycollinear with the axis (A) of the focussed beam 20. However, as theplasma builds the electron movement diverges from the axis (A) of thebeam 20 and rapidly spreads throughout the air/fuel spray 26. The freeelectrons are accelerated to higher energies, forming "hot" electrons,which extend over a large area of the combustion chamber 22. The plasmaof free electrons and ions that is generated within the combustionchamber 22 may have a temperature approaching 20,000 K.

As employed herein, a plasma is considered to be a gaseous regioncontaining free electrons and ions at a given density or concentration.Typical electron/ion densities associated with plasmas are in the rangeof 10⁸ cm⁻³ to 10¹⁰ cm⁻³. It has been found that the use of theinvention results in a plasma having significantly higher densities inthe range of 10¹⁶ cm⁻³ to 10¹⁸ cm⁻³. The pulse of laser energy ismaintained for pumping the developing plasma to at least a point whereatthe plasma becomes self-sustaining; that is, to a point where the plasmawill exist for a period of time without requiring input energy from thelaser beam. As employed herein, the point at which the plasma becomesself-sustained is considered to be at free electron energies in therange of approximately 20 electron volts (ev) to approximately 30 ev.

The plasma generation is believed to operate by a multiphoton processwherein bound electrons absorb several photons, each photon of lesserenergy than an ionization energy. For example, a laser beam ofapproximately one micrometer wavelength initially generates freeelectrons having energies of approximately one electron volt. Absorptionof several such low energy photons by an electron bound to an oxygenatom, within a relatively short period of time, results in the electronabsorbing sufficient energy to cause the atom to become ionized. Theresulting ionized oxygen atom thus contributes a free electron to theplasma.

The actual ignition of the air/fuel spray is believed to proceedprimarily by a diffusion and recombination process. The fully developed,self-sustaining plasma contains free electrons and ions. As the plasmabegins to cool, free electrons are captured by ions to create neutral,excited atoms. These neutral, excited atoms in turn create neutral,excited molecules/atoms which are the precursors to ignition. Therecombination process that results in ignition may require severalmicroseconds, as indicated by FIG. 2. At the time of ignition, theremaining free electrons are believed to have energies on the order ofapproximately one to approximately five electron volts.

Initially, it has been found that the fuel spray 26 appearssubstantially transparent to the laser beam 20, and relatively littleabsorption of the beam 20 occurs. However, as the plasma builds withinthe combustion chamber 22 the absorption Of the laser pulse 20 has beenfound to increase until, finally, the energy of the laser pulse 20 isalmost totally absorbed by the plasma. Laser energy absorptions of90-95% are typical for the fully developed plasma. This increasedabsorption further heats the plasma and "pumps" the plasma energy tohigher levels.

As was stated, for a laser with an approximately one micrometerwavelength the energy of the free electrons is initially approximatelyone electron volt. However, after the plasma is developed energies ofapproximately 30 electron volts may be achieved. In this regard it isnoted that energies of approximately 10 electron volts are required todisassociate a molecule, while 15 electron volts is sufficient to ionizeoxygen molecules. Thus, it can readily be seen that energiessignificantly above that required to ionize the oxygen molecules arepresent within the plasma of the combustion chamber.

As can be seen in FIG. 2, the pulse width of the incident laser beam 20is relatively short compared to the amount of time to achieve ignition.That is, the process of plasma generation occurs within several tens ofnanoseconds after the laser pulse is introduced into the combustionchamber 22. However, the actual, global ignition of the fuel spray 26may not occur for up to some number of microseconds. As a result, thepulse width of the laser radiation can be made significantly shorterthan the amount of time that elapses between plasma development and theignition of the air/fuel spray 26. That is, the pulse of coherentradiation is introduced into the combustion chamber to initiate plasmadevelopment, maintained at least until the plasma becomesself-sustained, and then removed, all within a period of time that isless than a time required to achieve ignition. This advantageouslyprovides for a low powered operation, relative to the linear systems ofthe prior art, and to the high powered system described by Smith in theabove-mentioned U.S. Pat. No. 4,302,933. As was noted above, Smithgenerates a LSA wave, that is, a propagating thermal wave. The laser ofSmith is thus required to maintain the plasma so as to provide thethermal wave.

More specifically, the generation of an LSA-wave is consistent with alonger pulsewidth (microsecond) laser source. In contradistinction, thepulsewidth taught by this invention, which is on the order ofnanoseconds, is sufficiently short such that the formation of anabsorption wave is unlikely. That is, the formation time of such a waveis believed to be much greater than the pulsewidth of the source 12.This invention requires only that the free electrons absorb the laserenergy. It is further believed that the subsequent electron motion isnot solely toward the laser source, as is indicated in U.S. Pat. No.4,302,933.

The ignition process utilizes the presence of free electrons. Theseelectrons may be naturally present, or may be the result ofphotoionization processes from gaseous or vapor species of the fueldroplet itself. As seen in FIG. 3, the droplet provides a region nearthe surface for which there is an enhancement of the electric fieldproduced by the laser source/fuel droplet interaction. This enhancementregion has an approximate thickness of 1 wavelength. Significantly,there is no requirement for an eye of a cyclonic air action as stated inU.S. Pat. No. 4,302,933.

As has been previously described, the teaching of this invention isdirected towards providing a low powered ignition system. This aspect ofthe invention is made apparent by a comparison with the system describedin U.S. Pat. No. 4,302,933. A minimum converging beam intensity that isdisclosed in 4,302,933 is 6×10⁸ W/cm² (utilizing a 10.6 micronwavelength).

FIG. 4 illustrates a focal region of a laser, where d is a minimumdiameter of a focal volume. A minimum intensity I_(min) is given by##EQU1## The minimum d is a function of the laser wavelength γ ##EQU2##where f is the focal length of a laser focusing element (lens) and D isthe diameter of the beam; i.e., f/D is optics controlled. Now, ##EQU3##Assuming that I_(min) is equal for both applications; i.e., the lowpowered system of this invention and the system disclosed in 4,302,933,then ##EQU4##

where γ(μm) represents wavelength.

Assuming a wavelength of γ≈1 μm for this embodiment of the invention,then

    P(10.6μ)≅100P(1μ).                         (5)

As can be seen, a minimum power required by the system disclosed in4,302,933, operating at 10.6 μm is approximately 100 times as large asthat required for the source 12 operating at approximately 1 μm.

In accordance with a method of this invention for igniting an air/fuelspray that is comprised of fuel droplets, the following steps areaccomplished: (a) providing the air/fuel spray within a combustionchamber; (b) introducing at least one pulse of coherent radiation intothe air/fuel spray, the at least one pulse interacting with freeelectrons and initiating a development of a plasma within the air/fuelspray; (c) maintaining the at least one pulse of coherent radiation forpumping the developing plasma to higher energies; and (d) terminatingthe at least one pulse of coherent radiation at a time after the plasmahas reached a predetermined energy and before a time that ignition ofthe air/fuel spray occurs.

As can be appreciated, the laser beam should exhibit certain preferredcharacteristics to achieve low powered, non-linear ignition. Thesecharacteristics of the laser beam include the coherency of the laserbeam, the pulse width, and the: power density.

The index of refraction of the fuel to be ignited is also aconsideration, in that both the electric field strength and electricfield shape external to the droplet are known to be a function of theindex of refraction of the droplet at the laser wavelength.

In accordance with an embodiment of the invention for use with, byexample, a fuel such as JP-4, JP-5, or JP-8; the laser 12 includes aNd:YAG laser having a wavelength of 1.064 micrometers, or the frequencydoubled (0.532 micron), or frequency tripled (0.355 micron) wavelength.That is, the wavelength is within a range of approximately 0.3micrometers to approximately 1.1 micrometers. The coherency of the beamis a consideration, in that a multimode beam generates interferenceeffects within the combustion chamber due to mutual interference of thedifferent wavelengths. As a result, any droplets located within a fringeregion wherein the laser power is minimized due to destructiveinterferences may not experience sufficient energy to initiate thenon-linear ignition process. Thus, a single mode, or approximatelysingle mode, beam is preferred, although not absolutely required. Apreferred pulse width for the Nd:YAG laser is within a range ofapproximately five to approximately 50 nanoseconds, and the powerdensity is within a range of approximately 10⁷ to approximately 10⁸W/cm². The coherency is assumed to be such that the laser light pulseenergy can be focussed to a beam cross-section, circular area oftypically 100 microns in diameter.

It is noted that the process of the invention is not wavelengthcritical, but is wavelength sensitive. That is, in contradistinction tothe selection of an electromagnetic wavelength that is highly absorbedby the particular fuel that comprises the droplet, as is done for alinear ignition system such as that described in the above referencedU.S. Pat. No. 4,947,640, the selected wavelength may be employed with anumber of different types of fuels.

In this regard it is also noted that although the primary ignitionmechanism is the non-linear process, employing the electric fieldoutside of the fuel droplet, some absorption and localized heating ofthe fuel droplets may occur within the fuel spray. For increased laserfluence (radiation energy per area) directed to the combustor, linearheating effects may dominate the non-linear effects and the system mayrevert to a linear ignition system. However, in that it is desirable tominimize the fluence to the combustion chamber 22 to provide low poweredoperation, the ignition process is typically and preferably dominated bythe non-linear process described herein.

It is within the scope of the invention to employ a fuel additive, suchas a low ionization threshold compound. In that only a very fewelectrons are required to initiate the ignition process, due to thepumping of the plasma and the free electron avalanche that resultstherefrom, only trace quantities of a photoemissive compound may berequired to be added to the fuel. This is advantageous in that the fuelproperties are not significantly affected by the addition of only tracequantities.

Although described in the context of a combustion chamber for a gasturbine engine it should be realized that the teaching of the inventionis applicable to combustion chambers in general wherein fuel dropletsare provided. The invention may also be employed with a wide range offuels including, but not limited to, JP-4, JP-5, JP-8 and diesel atvarying fuel/air mixtures. Also, the teaching of the invention is notintended to be limited to any one particular wavelength or any oneparticular power level, power density, pulse width, or pulse repetitionrate.

Thus, while the invention has been particularly shown and described withrespect to an exemplary embodiment thereof, it will be understood bythose skilled in the art that changes in form and details may be madetherein without departing from the scope and the spirit of theinvention.

What is claimed is:
 1. An ignition system for igniting, within acombustion chamber, an air/fuel spray comprised of fuel droplets, theignition system comprising:means for generating at least one pulse ofcoherent radiation; means for directing the at least one pulse into theair/fuel spray for initiating a development of a plasma within theair/fuel spray; said generating means including pulse control meanscomprising means for maintaining the pulse of coherent radiation forpumping the developing plasma and means for terminating the pulse ofcoherent radiation at a time after the plasma has become self-sustainedand before a time that an ignition of the air/fuel spray occurs, theignition using ignition precursors that are created from theself-sustained plasma after the termination of the pulse; and feedbackmeans including sensor means for detecting a presence of a flame withinthe combustion chamber, an output of said sensor means being coupled tosaid generating means for causing said generating means to repetitivelygenerate pulses of coherent radiation at least until a presence of aflame is detected by said sensor means.
 2. An ignition system as setforth in claim 1 wherein said means for directing includes means forfocussing the at least one pulse of coherent radiation within theair/fuel spray.
 3. An ignition system as set forth in claim 1 whereinthe at least one pulse of coherent radiation has a wavelength within arange of approximately 0.3 micrometers to approximately 1.1 micrometers,a pulse width within a range of approximately 5 nanoseconds toapproximately 50 nanoseconds, and a power density within a range ofapproximately 10⁷ W/cm² to approximately 10⁸ W/cm².
 4. An ignitionsystem as set forth in claim 1 wherein said means for directing iscomprised of an optical fiber having a first end that is opticallycoupled to an output of said generating means and a second end that isoptically coupled to said combustion chamber.